A continuous-jet ink-jet printhead consists of a device which creates a train of droplets and devices for steering and catching these droplets. The devices for steering and catching the droplets control which droplets impact upon the surface being printed and, in some cases, control the impact position of the droplets. In continuous-jet printing, a stream of liquid is forced out of a nozzle under pressure in a continuous stream. Such a stream has a natural tendency to break up into a stream of droplets in order to reduce the surface energy of the stream. This break-up is most favorable at a particular frequency. For invicid fluids, this frequency is called the Rayleigh frequency and is given by: ##EQU1## where V is the velocity of the fluid in the jet, D is the diameter of the nozzle and the frequency is in Hertz.
In the absence of an externally applied perturbation, the stream will break up at a frequency near to the Rayleigh frequency, but with irregularly sized droplets. In order to create a regular break up, the standard practice is to apply a regular perturbation to the stream at or near the Rayleigh frequency. The droplets are then formed at the same frequency as the perturbation applied to the stream. A common problem with this approach is the formation of satellite droplets which are small, undesired droplets which can form between the larger, desired droplets as a by-product of the formation of these larger droplets. As the filamentary region between the large droplets decreases in diameter and a large droplet breaks free of the stream, the filamentary region can itself detach and collapse into a small droplet, thus forming a satellite droplet. The details of the shape of the filamentary region during droplet detachment are crucial to avoiding or controlling such satellite droplets.
The most common method for creating a perturbation to the stream is to use a piezo active material which is excited by an oscillating electrical signal at the desired frequency, resulting in a mechanical motion of the piezo material. In most droplet generators the piezo device is mounted to a structure which is caused to vibrate and this structure in combination with the piezo has a mechanical resonance at the desired frequency of operation. By such resonant operation a large amplitude of oscillation of the mechanical structure may be gained. In one type of resonant droplet generator the mechanical resonator is of the form of a rod which is caused to vibrate up and down within a fluid cavity and with its end some distance above the entrance to a nozzle orifice. The motion of the rod causes a perturbation to the stream by acoustic coupling through the fluid and this acoustic coupling must be designed.
In a second type of resonant droplet generator the body of the droplet generator is itself the mechanical resonator. The resonator body is fabricated of stainless steel and the orifices are in a linear array on an orifice plate which is bonded to the resonator body with adhesive. The orifice plate is typically made of nickel by an electroforming process. In this case, the motion of the resonant structure causes the orifice itself, which is bonded to the bottom of the resonant structure, to vibrate up and down, and by this primary mechanism, a regular perturbation is applied to the stream. Such generators may also have the undesirable effect of coupling the vibration of the body to the stream through the fluid within the body.
A common problem with all excitation driven droplet generators is the formation of satellite droplets. Satellite droplet formation is both the consequence of jet stream perturbations prior to break up and of fluid properties such as viscosity and surface tension and specific gravity during drop separation. Common water and solvent based inks can vary in density from 0.85 to 2.0 gm/cm3, with viscosities varying from 1 to 10 centipoise and surface tension varying from 20 to 80 dyne/cm. Flow rate, excitation frequency and wave form, drive amplitude, and temperature are among the variables which affect the formation of droplets during break-off. However, if (as is conventional practice) the excitation frequency approximates the natural or Rayleigh frequency for a given orifice and flow rate, then satellites for any Newtonian fluid are known to appear in the following sequence as a function of the energy applied to that nozzle in the form of piezo excitation voltage. At the lowest level of excitation necessary to initiate drop formation, "slow" or rearward merging satellites will appear (rearward merging satellites will move toward and join the main droplet which is closer to the nozzle--to the rear in the direction of flow). During this mode of operation, unwanted charge variations occur as rearward merging satellites transfer part of the charge from their parent forward drop to the following drop when merging occurs. As the excitation level increases, intermediate or infinite satellites appear which do not merge at all, and depending upon the application, can interfere with proper printing. Further increase in the piezo drive amplitude will result in "fast" or forward merging satellites (forward merging satellites will move toward and join the main droplet which is further from the nozzle--forward in the direction of flow). Forward merging satellites produce a satellite free stream entering the deflection field and permit precise placement of drops on the print substrate. Additional increase in piezo excitation will precipitate a condition known in the art as "drop separation fold back" accompanied by a lengthening of break-off and phase sequence reversal, ie. an abrupt transition from forward to rearward merging. Virtually all nozzles used for continuous-jet ink jet printing exhibit these satellite patterns and piezo drive is the principal variable governing the resolution of satellites into rearward, forward or infinite modes. Furthermore, there is a direct relationship between excitation frequency and excitation drive. Within certain limits, any departure from the Rayleigh frequency can be compensated with increased excitation. Conversely, any approximation to the natural break-off frequency will create the same breakoff condition with reduced excitation.
Problems arise for droplet generators based on mechanical and/or fluid resonance because their ability to impart energy to the jet stream is narrowly frequency dependent. Small departures from the tuned frequencies of these devices require large increases in the drive voltage required to maintain the same level of excitation. As a result, a given mechanically resonant device can operate over only a small range of nozzle size and flowrate. The useful stable flowrate range of operation for a given orifice is much broader than the range for drive compensation in a mechanically resonant droplet generator. Fluid resonant devices encounter difficulties accommodating the wide range of acoustic properties of water and solvent based inks, and any harmonic or attenuating effects of operating off frequency.
In the prior art, efforts have been made to attenuate all resonances by coupling the excitation source and fluid through "acoustically soft" materials as in U.S. Pat. No. 4,727,379. Since this device is non-resonant over its entire useful range of operation, it is capable of ranging in frequency in order to support the Rayleigh frequency range of any given orifice and flowrate. However, this device relies on the acoustic properties of the fluid to impart energy to the jetting stream, which makes it fluid specific with respect to the geometry of the fluid cavity. If the energy which is transmitted through the fluid conduit directly to the fluid stream does not closely match the fluid resonance frequency of the ink cavity then anti-resonances result which are destructive to the desired standing wave at fluid resonance. The drive voltage necessary to operate a nozzle in which the excitation energy is fluid coupled rises rapidly in the region between resonance and anti-resonance. Small changes in fluid properties or in frequency drift can result in a shift between fluid resonance and anti-resonance. U.S. Pat. No. 5,063,393 is a later refinement in which a hybrid multi-resonant fluid chamber is fabricated from "acoustically soft" materials in order to support several fluid resonances in the region of the system operating frequency. Through this means the nozzle response is flattened, and the region of forward merging satellite free operation is broadened to include commonly used inks with differing acoustic properties. Commonly used water and solvent based inks have sound propagation rates ranging from 1000 to 1650 m/s.
U.S. Pat. No. 5,196,860 provides a nozzle control system which monitors the condition of satellite drops and the drop break-off point in order to compute and apply a satisfactory range of nozzle drive voltages that will maintain operation of an ink jet printer in the favorable region between forward merging satellites and drop separation fold back, regardless of ink type and temperature.
In a "Rapid Prototyping" technique know as Three Dimensional Printing described in U.S. Pat. No. 5,204,055 a component is defined on a layer-by-layer basis. Each cross section of a CAD model is created by spreading a thin layer of powder or particulate material in a predefined region, usually over the top of a piston. The pattern within the layer is created by ink-jet printing a binder material which serves to bind or bond the particles in an intended pattern. Successive layers of particles are applied and selectively bonded within and between layers to build up, layer by layer, a three dimensional structure. This technique is used for rapid prototyping of tooling and molds and other objects, and can also be used for producing finished production objects.
The nature of the binder material varies widely from application to application and often includes the use of colloids and slurries of particles in suspension. For example, colloidal silica binder is printed into alumina powder to create ceramic molds for metal casting. Binders of a wide range of chemical species may be printed as well, ranging from highly acidic to highly basic aqueous media and including a wide range of organic solvents such as alcohols, acetone and chlorinated solvents such as chloroform.
In a typical arrangement, the powder layers are spread over the top of a piston and the piston is lowered one layer thickness prior to the spreading of each layer, thereby maintaining the top of the bed of powder at a specified height. The ink-jet printhead is typically a continuous-jet type of printhead with an array of nozzles arranged in a row. The printhead is mounted to a carriage which is moved over the powder bed in a raster pattern. The combination of this raster pattern with the ability to turn the printhead on and off, defines the pattern of each layer. The raster scanning pattern is achieved by a reciprocating motion on one axis and this axis is then translated over the powder bed to achieve the second axis of the raster pattern. Typically, the printhead is accelerated at 20 m/sec.sup.2 (2 g's) during the turn-around portions of the reciprocation and moves at a velocity of 1.5 meter/sec as it sweeps across the powder bed.
In view of the particular requirements on ink-jet printing of the Three Dimensional Printing process, the standard methods of construction of droplet generators are unsatisfactory in a number of key regards.
The materials of construction of current droplet generators are often limited by the need to achieve particular geometries and the wetted materials may be incompatible with particular binders. For example, one droplet generator employs an electroformed nickel orifice plate which will not tolerate corrosive or abrasive binders. Some of the highly acidic binders will corrode stainless steel, the main material of construction of most droplet generators. Further, the typical construction involves the use of gaskets, seals and adhesives in wetted contact with the binders, all of which pose serious problems to handling a wide range of binder materials.
The typical construction uses a chamber and such chambers can have regions of fluid stagnation where little or no fluid flow takes place. Such stagnant fluid flow can lead to nucleation, agglomeration or settling of colloidal or slurry based binders. Stagnant regions can lead to difficulties in cleaning a fluid out of a chamber or in switching from one binder type to another without a potentially undesired mixing of the two binder types. In order to scale up the production rate of the Three Dimensional Printing process it will be desired to create printheads and droplet generators with operative widths as large as the powderbed, potentially several meters in extent. In the design and manufacture of droplet generator based on mechanical resonance, great care must be exercised in order to avoid unwanted mechanical resonances. This problem is especially acute in printheads of greater widths and may become practically impossible for the widths of printhead needed for the Three Dimensional Printing process. A droplet generator based on a mechanically resonant structure must be operated at its resonant frequency and can become inoperative with even small differences in frequency away from the resonant frequency. However, as the carriage which creates the raster motion of the printhead over the powderbed will inevitably have some small variations of velocity as it sweeps over the powderbed, it may be desirable to slightly vary the frequency of the droplet generation so as to maintain synchrony of droplet generation with position of the printhead over the powderbed.
In most applications of continuous-jet ink-jet printing, the printhead is substantially stationary and the paper or other product on which ink is to be deposited is transported past the stationary head. Thus the design of conventional printheads for ink jet printers need not accommodate a transport mechanism for moving the head across a working surface and the printhead is often quite large and massive. As noted, In the Three Dimensional Printing process, the printhead is often reciprocated at high speeds and high accelerations. Thus, printheads of the usual construction would severely limit the width of printhead that could be used, simply due to the size and especially the mass of the printhead.
Adapting a commercial printhead intended for stationary operation also presents problems for fluid support. Pressure surges can result from the acceleration of fluid in nozzle chambers and manifolds in direct communication with orifices and cause fluctuations in stream break-off and phase stability. The serviceability of large numbers of nozzles in one unit is extremely difficult when jetting materials with a tendency to settle, agglomerate or nucleate. Any problems associated with the failure of a single jet place the entire array at risk since disassembly and cleaning procedures tend to dislodge particulates from seals and recessed areas and cleaning solutions often react with residual printing fluids. Attempts to restart a multiple jet printhead of this type are usually difficult.